1 files changed, 68 insertions, 0 deletions
diff --git a/searchlib/src/tests/index/field_length_calculator/field_length_calculator_test.cpp b/searchlib/src/tests/index/field_length_calculator/field_length_calculator_test.cpp
new file mode 100644
index 00000000000..c99d241cbc0
--- /dev/null
+++ b/searchlib/src/tests/index/field_length_calculator/field_length_calculator_test.cpp
@@ -0,0 +1,68 @@
+// Copyright 2019 Oath Inc. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+
+#include <vespa/searchlib/index/field_length_calculator.h>
+#include <vespa/vespalib/gtest/gtest.h>
+
+using search::index::FieldLengthCalculator;
+
+namespace search::index {
+
+namespace {
+
+// Arithmetic average of arithmetic sequence 1, 2, ... , samples
+double arith_avg(uint32_t samples) {
+    return static_cast<double>(samples + 1) / 2;
+}
+
+}
+
+TEST(FieldLengthCalculatorTest, empty_is_zero)
+{
+    FieldLengthCalculator calc;
+    EXPECT_EQ(0.0, calc.get_average_field_length());
+    EXPECT_EQ(0, calc.get_num_samples());
+}
+
+TEST(FieldLengthCalculatorTest, startup_is_average)
+{
+    FieldLengthCalculator calc;
+    calc.add_field_length(3);
+    EXPECT_DOUBLE_EQ(3.0, calc.get_average_field_length());
+    EXPECT_EQ(1, calc.get_num_samples());
+    calc.add_field_length(4);
+    EXPECT_DOUBLE_EQ(3.5, calc.get_average_field_length());
+    EXPECT_EQ(2, calc.get_num_samples());
+    calc.add_field_length(7);
+    EXPECT_DOUBLE_EQ((3 + 4 + 7)/3.0, calc.get_average_field_length());
+    EXPECT_EQ(3, calc.get_num_samples());
+    calc.add_field_length(9);
+    EXPECT_DOUBLE_EQ((3 + 4 + 7 + 9)/4.0, calc.get_average_field_length());
+    EXPECT_EQ(4, calc.get_num_samples());
+}
+
+TEST(FieldLengthCalculatorTest, average_until_max_num_samples)
+{
+    const uint32_t max_num_samples = 5;
+    FieldLengthCalculator calc(0.0, 0, max_num_samples);
+    static constexpr double epsilon = 0.000000001; // Allowed difference
+    for (uint32_t i = 0; i + 1 < max_num_samples; ++i) {
+        calc.add_field_length(i + 1);
+    }
+    // Arithmetic average
+    EXPECT_NEAR(arith_avg(max_num_samples - 1), calc.get_average_field_length(), epsilon);
+    EXPECT_EQ(max_num_samples - 1, calc.get_num_samples());
+    calc.add_field_length(max_num_samples);
+    // Arithmetic average
+    EXPECT_NEAR(arith_avg(max_num_samples), calc.get_average_field_length(), epsilon);
+    EXPECT_EQ(max_num_samples, calc.get_num_samples());
+    calc.add_field_length(max_num_samples + 1);
+    // No longer arithmetic average
+    EXPECT_LT(arith_avg(max_num_samples + 1), calc.get_average_field_length());
+    // Switched to exponential decay
+    EXPECT_NEAR((arith_avg(max_num_samples) * (max_num_samples - 1) + max_num_samples + 1) / max_num_samples, calc.get_average_field_length(), epsilon);
+    EXPECT_EQ(max_num_samples, calc.get_num_samples());
+}
+
+}
+
+GTEST_MAIN_RUN_ALL_TESTS()